window.frames[i] window.frames.length frames[i] frames.length
Though the Frame object is sometimes referred to, there is, strictly speaking, no such object. All frames within a browser window are instances of the Window object, and they contain the same properties and support the same methods and event handlers as the Window object does. See the Window object, and its properties, methods, and event handlers for details.
There are a few practical differences between Window objects that represent top-level browser windows, and those that represent frames within a browser window, however: